— reviewed against the regs as they stand.
GridMonkey — free in your browser
See what the grid is actually running on right now
The live generation mix, the carbon intensity readout, the 24-hour trend and the breakdown by generator — the real-world context behind the electricity factor SAP applies.
Completely free. No sign-up, no ads, no catch.
Every fuel carries a carbon factor
SAP converts every kWh your dwelling uses into CO₂ and primary energy using fuel-specific factors. Your target (TER) is set by a notional dwelling using mains gas — so fuels with worse factors than gas start at a disadvantage that must be clawed back elsewhere.
The SAP 10 shake-up
Under SAP 2012, grid electricity carried a carbon factor 2.4× that of mains gas — making all-electric new builds notoriously hard to pass. SAP 10 cut electricity’s factor from 0.519 to 0.233 kgCO₂/kWh, and SAP 10.2 — the version Part L 2021 is assessed with — took it to 0.136, below gas (0.210) for the first time; SAP 10.3, the Future Homes Standard methodology, goes to 0.086 against 0.214 for gas (what changed). Direct electric heating is now viable in the calculation — and a heat pump, delivering 3+ units of heat per unit of electricity, comfortably beats gas on carbon. One caution: these are SAP’s own factors, fixed for the life of a SAP version. The DESNZ and Defra factors used in energy statements and SECR reporting are updated annually and are different numbers for a different job — the building-relevant set is here.
Practical implications
Mains gas remains the baseline the targets assume — but with the Future Homes Standard phasing fossil-fuel heating out of new builds, it’s a shrinking option. Heat pumps are now usually the strongest choice in SAP terms. Direct electric (panel heaters, electric boilers) passes more easily than before but high running costs still show in the calculation. LPG and oil carry worse economics and carbon, and typically need fabric or renewable compensation.
One caveat worth holding on to: the electricity factor SAP uses is a single annual average, and the real grid swings a long way either side of it through the day — GridMonkey shows what Britain is generating right now if you want to see the spread behind the number.
Choosing fuel early — before the design-stage SAP — avoids expensive redesign. If you’re undecided, we can model the options side by side. Ask us.